If you quieten someone or something, or if they quieten, you make them become less noisy, less active, or silent. She tried to quieten her breathing A man shouted and the dogs suddenly quietened
To quieten fears or complaints means to persuade people that there is no good reason for them. Russian intelligence will take a long time to quieten the paranoia of the West
If someone or something quietens down or if you quieten them down, they become less noisy or less active. The labour unrest which swept the country last week has quietened down Somehow I managed to quieten her down Tom's words before the match might also have quietened down our own supporters. = calm down